The invention relates to an optical system, in particular a microlithographic projection printing installation, in particular having a slot-shaped image field or rotationally non-symmetrical illumination,
a) having a light source;
b) having at least one optical element, in particular a lens or a mirror, which
ba) comprises at least one surface acted upon by radiation of the light source; and
bb) the basic shape of which at least in the region of the at least one surface acted upon by radiation is substantially symmetrical relative to an axis of rotational symmetry.
The invention further relates to methods of compensating rotationally non-symmetrical image defects in such an optical system, through which a light beam is passed.
The imaging quality of such an optical system is often impaired by rotationally non-symmetrical image defects. Such image defects arise, for example, as a result of rotationally non-symmetrical heating of the at least one optical element of the optical system or as a result of other effects, such as e.g. compaction, which lead to a corresponding rotationally non-symmetrical expansion and/or refractive index distribution in the optical element. Such rotationally non-symmetrical heating may be lumination-induced but may also have other causes, e.g. a rotationally non-symmetrical thermal coupling of the optical element to its surroundings or some other rotationally non-symmetrical thermal influence upon the optical element. Other causes of rotationally non-symmetrical image defects may be, for example, material irregularities or variations in the shape of the irradiated surface of the optical element.
The object of the present invention is to compensate and/or render symmetrical rotationally non-symmetrical image defects of the at least one optical element of the optical system in order to improve the imaging quality.
Said object is achieved according to the invention in that
c) the optical element or its housing is rotatably connected to a frame by at least one bearing;
d) an actuator is provided, which sets the optical element or its housing in rotation about the axis of rotational symmetry; wherein
e) the actuator cooperates with a control device, which activates the actuator at least temporarily during the period, when the optical element is exposed to lumination, for rotation of the optical element.
Where the rotationally non-symmetrical image defects are light- or heat-induced, their very formation may be prevented or at least reduced by the arrangement according to the invention through rotation of the optical element. The rotation in said case has to be effected within a time which is short, measured against the time constant of the formation of lumination-induced image defects.
Where rotationally non-symmetrical image defects are not light- or heat-induced, they may be compensated by the arrangement according to the invention provided that the rotation is effected quickly compared to the lumination time.
This leads, as a whole, to rotationally at least substantially symmetrical imaging properties. By virtue of rendering the imaging properties symmetrical in said manner, either the optical system has an adequately good imaging quality without further corrections or a relatively simple correction with additional rotationally symmetrical optical correction elements becomes possible.
The actuator may be formed by an electric drive having a frame-fixed stator and a rotor non-rotatably connected to the optical element, wherein an air gap remains between the rotor and a stator non-rotatably connected to the frame. Such an electric drive allows precise rotation of the optical element without causing vibrations.
Electrical equipment which co-rotates with the optical element may be connected by cable to a power supply. Such a power supply, which is naturally possible only if the optical element does not rotate always in one direction of rotation, is inexpensive.
Alternatively, electrical equipment which co-rotates with the optical element may be connected by an inductive coupling device to a power supply. Such a power supply is possible also with an optical element, which rotates always in one direction of rotation, and unlike sliding contacts, which are equally possible in such a situation, is wear-free. The possibility of vibrations being transmitted via a sliding contact to the optical element and impairing its imaging quality also does not apply.
At least one bearing may be a magnet bearing. Such bearings may be used for contactless support, in which case vibrations during rotation are likewise avoided.
Alternatively or additionally at least one hydrostatic bearing or an air bearing may be provided. With such bearings also, hardly any vibrations or no vibrations at all are transmitted during rotation.
Instead of the electric drive, the actuator may be formed by a mechanically operating drive with a gear. Such an actuator is inexpensive and, independently of the manufacturing tolerances, likewise enables precise rotation of the optical element.
When a worm gear is used as a gear, it is possible with a relatively high driving speed to achieve a slow rotation of the optical element. This is beneficial for isolation of the optical element from the drive vibrations.
Alternatively, the gear may be a bevel gear. With such a gear it is possible to realize high rotational speeds during rotation of the optical element so that, even in the case of a short lumination time, image defects may be rendered symmetrical in accordance with the invention.
In a preferred form of construction, at least one optical element, which is held with further optical elements in a common housing, is rotatable relative to the housing. The optical elements which present the most pronounced rotationally non-symmetrical image defects may be selected to be rotatable. The other optical elements in the housing may be of a non-rotatable design, thereby increasing the stability of an optical system which comprises said optical elements. In particular, the housing may be of a non-rotatable design, thereby avoiding the supply line connection problems which might arise with a rotating housing. An additional advantage arises from the fact that through the rotation of one optical element image defects of other, non-rotated optical elements may be compensated.
Alternatively, a housing containing a plurality of optical elements may be provided, which is rotatably connected to a frame. In said case, all of the optical elements inside the housing are rotated with the latter. The optical system composed of said optical elements may therefore be inherently very stable.
A further object of the invention is to develop a method of the type described initially in such a way that rotationally non-symmetrical image defects are rendered symmetrical in a particularly effective manner.
Said object is achieved in a variant of the method according to the invention by the following process steps:
a) rotation of the optical element through a specific angle of rotation;
b) reverse rotation of the optical element through a specific angle of reverse rotation.
In most cases, continuous rotation of the optical element in one direction of rotation is not necessary to render rotationally non-symmetrical image defects symmetrical. When the optical element is merely rotated back and forth through specific angles of rotation, the optical system may be of a mechanically simplified design. In addition, supplying power to electrical equipment co-rotating with the optical element is simplified because, as a rule, trailing cables may be used.
The control device may activate the reversal points between the two directions of rotation within an angle-of-rotation range at an identical relative rate of occurrence. This prevents the optical element from changing its direction of rotation always at the same reversal point, which may result in unilateral loading of the optical system.
The optical element may be rotated in relation to a starting position in accordance with the symmetry of the radiated power distribution in the light beam through a fraction of a full revolution. The higher the order of said symmetry, the lower the angle of rotation needed to render the image defects symmetrical. And the mechanical design and the power supply of the optical system become correspondingly simpler.
An alternative method of compensating rotationally non-symmetrical image defects in accordance with the invention comprises the following process steps:
a) compared to the time constant of the formation of image defects induced by the radiation of the light source, fast rotation of the optical element through a specific angle of rotation;
b) maintaining of the position of the optical element for a dwell period, which is long compared to the duration of rotation and short compared to the time constant of the formation of image defects induced by the radiation of the light source;
c) compared to the time constant of the formation of image defects induced by the radiation of the light source, fast reverse rotation of the optical element through a specific angle of reverse rotation.
Such a rotation sequence for compensating lumination-induced image defects leads, assuming a suitable symmetry of the image defects, to a further reduction of the angle of rotation needed to produce rotationally substantially symmetrical imaging properties of the optical system. The effect achieved by rotating the optical element at a speed which is fast compared to the dwell period is that imaging changes of the optical element, which are caused by irradiation of the regions of the optical element which are irradiated only during rotation, are of no consequence compared to imaging changes, which arise in the regions of the optical element which are irradiated during the dwell period.
The image defects induced during the dwell period in the extreme positions between rotations cancel each other out, given suitable adaptation of the dwell time and of the angle of rotation to the time constant of the formation of the lumination-induced image defect and to the symmetry of the latter, in such a way that, on the whole, rotationally substantially symmetrical imaging properties result.
The optical element may be rotated in relation to a starting position in accordance with the symmetry of the radiated power distribution in the light beam through a fraction of a full revolution. With the alternative method also, given multiple-order symmetry of the lumination-induced image defects, the already previously described additional reduction of the absolute angle of rotation is possible.
In a further alternative method of compensating rotationally non-symmetrical image defects in accordance with the invention, the optical element is rotated at a preset rotational speed. Such rotation is possible with a simple drive controller.
The optical element, while being acted upon by the light beam, may be rotated through a plurality of revolutions. By said means image defects, which are formed in a light-induced manner with a relatively short time constant or are not light-induced, are also rendered symmetrical. Even a plurality of mutually independent, rotationally non-symmetrical image defects are reliably rendered symmetrical by said means.